This invention relates a method for forming a film.
A film-forming technique plays an important part in development of a material and a device. In the recent very large integration taking advantage of a microfabrication technique, it is desired to establish a new film-forming technique particularly in an electronic device such as a ULSI. Up to now, the CVD methods, in which all the constituting elements of a film to be formed on a substrate are supplied from an external atmosphere or the thermal oxidizing method in which elements from an external atmosphere are reacted with constituting elements of a substrate to form a film, have been mainly used. In both of the above methods, now, the elements from the external atmosphere are introduced into a vacuum vessel in molecularity.
The recent miniaturization of an elemental device restricts its film-forming process, particularly requiring to lower its processing temperature. One of the factors to make higher the processing temperature is that the constituting elements from the external atmosphere are supplied in molecularity. That is, a part of atoms constituting the molecular elements to be supplied or dissociated atomicity elements from the molecular elements are essentially required in the film-forming process. The conventional film-forming technique dissociates the supplied molecular elements near a heated substrate, so requiring the energy of the dissociation for the temperature of the heated substrate for. Therefore, it has its limit by itself in lowering of the film-forming process.
In the above film-forming technique of supplying the constituting elements from the external atmosphere and depositing the elements on the substrate, for lowering the processing temperature, a sputtering method or a plasma CVD method, in which a given plasma is employed, is suggested and practically used in a part of the film-forming process. The former method etched a solid target by using a plasma energy and deposits the etched particles on a given substrate and the latter method dissociates raw material gas to be supplied and deposits the dissociated elements on a given substrate. These methods are prevailing in solving the above problem, in the view of supplying in advance dissociated elements from the raw material gas onto the given substrate.
On the other hand, a thermal oxidizing process of a silicon substrate, which is typical in the above method of reacting the elements supplied from the external atmosphere with the constituting elements of the substrate, have been widely used in the forming process of gate oxide films of MOSFETs. In the thermal oxidizing process, the gate insulating films having good qualities can be easily formed by heating and holding the silicon substrate at 800xc2x0 C. and over under an oxidizing atmosphere (oxygen molecule-atmosphere). The obtained silicon oxide film is generally called as a xe2x80x9cthermally oxidized filmxe2x80x9d. The above method is described in xe2x80x9cJ.Appl.Physxe2x80x9d, p3770, No. 36 (1965), by B. E. Deal and A. S. Grove, and xe2x80x9cQuick Reference Manual for Silicon Integrated Circuit Technologyxe2x80x9d by W. E. Beadle, J. C. C Tsai and R. D. Plummer, published by xe2x80x9cJohn Wiley and Sons Co.(1985), etc. The main reason of using the high temperature and the large excited energy process is that the silicon oxide film/silicon substrate-boundary surface exhibits good electric characteristics.
Although many methods to form the silicon oxide film on the silicon substrate at a low temperature such as the sputtering method or the plasma CVD method as a directly depositing method, are made an attempt, generally, they shows extremely low boundary face-level density (Dit) which is a typical reference mark for the boundary face characteristic. The reason is that the dangling bonds near the silicon substrate surface, which directly influence the Dit, remain after the silicon oxide film/silicon substrate-boundary face is formed. The part of the dangling bonds may be terminated by hydrogen atoms in a CVD method, but the silicon atom/lydrogen atom-bonds are often cut easily at the ensuing process requiring a temperature of about 400xc2x0 C. Accordingly, the low temperature-forming method of the silicon oxide film lacks a long-term reliability and has trouble with being applied to forming gate oxide films of LSIs.
Moreover, the method of directly introducing elements dissociated in a plasma atmosphere from an external atmosphere and reacting the dissociated elements with the constituting atomic elements of a substrate is made an attempt to lower the processing temperature. However, it is known, when many molecules each composed of plural atoms are introduced in a plasma, the plasma has an extreme wide energy distribution and thus, the molecules are transformed into a variety of activated species including molecular ions. The thus obtained film does not have its good quality, so that the above method is almost never employed in forming the gate oxide films of MOSFETs requiring harsh conditions.
In addition to the silicon oxide material, a silicon nitride material may be used for the gate oxide film or a passivative film which is insulating film. The silicon nitride film is formed by a variety of methods as in the silicon oxide film. In the case of forming the gate oxide film of the silicon nitride, since the film undesirably has many boundary face-level at its silicon/silicon nitride-boundary face, the film is generally formed so as to have a silicon/silicon oxide/silicon nitride-boundary face.
Recently, a successive process in forming an elemental device requires low temperature process intensely. To comply with the request, the lowering technique of the processing temperature is desired in the whole film-forming process.
In these years, the miniaturization of the MOSFETs and the lowering of the voltage of a driving power supply reach their limits, so that the conventional thermally oxidized film can not give the MOSFETs sufficient qualities. One of the reasons occurs from the high temperature thermal treatment at 800xc2x0 C. for several ten minutes. That is, when the miniaturization requires to control the impurity-profiles of the MOSFET semiconductor precisely within their shallow profiles, the high temperature thermal treatment destroys the precise shallow impurity-profiles. As mentioned above, the CVD method or the sputtering method not requiring the high temperature thermal treatment degrades the insulating characteristics and the boundary face-qualities because of many dangling bonds. As a result, the miniaturization of the MOSFETs can not tolerate the high temperature thermal treatment, so that the insulating film having good qualities can not be obtained.
Besides, there is a problem due to the environmental change around the MOSFETs. The use of a wafer having a large size for developing productivity has to satisfy the uniformity of the characteristics in all the MOSFETs entirely on the wafer surface. In the case of forming oxide films in a large size equipment corresponding to the large size wafer by using the conventional thermally oxidizing method, the relatively large activation energy of about 1.1 eV to oxidize the wafer surface changes the rate of reaction due to the temperature fluctuation during the heating. It means the difficulty in obtaining oxide films having their uniform thickness on the wafer. When complex calculations are carried out by increasing the number of the MOSFET per one chip, the fluctuation of the characteristics in the MOSFETs becomes not tolerated and severe. Therefore, the insulating films having uniform characteristics has to be formed for many MOSFETs on the wafer.
The adoption of the insulating film formed at a low temperature for the gate oxide film requires to reduce its Dit value, but now, the high temperature process is required for maintaining the electric characteristic of the gate oxide film. Although the high temperaturexe2x80x94and large activation energy-process have been used for giving preference to the electric characteristics under the conditions of the use of a small size wafer and not proceeded microfabrication, a low temperaturexe2x80x94and small activation energy-process is required for more miniaturization and enlarging the wafer size without the electric characteristics.
For realizing the low temperature in the film-forming process, it is conceivable to dissociate the molecular elements constituting the film in atomicity and supply the dissociated elements. On the other hand, the molecular elements, if they have their excited energy states from their ground states as their molecularity states, have their excited states maintaining their molecularity states (molecular excitation-state), their ionized states maintaining their molecularity states (molecular ionization-state) and their dissociated states perfectly in atomicity (atomicity-state). When an energy is supplied to the molecular elements from a plasma, they have the above states by low energy turn. Accordingly, when the molecular elements are excited to the atomicity-state, for example, they necessarily have another low energy state. Moreover, when they have large energy to be excited to the atomicity-state, they are almost never excited to the atomicity-state.
According to the dissociating method of the molecular elements to the atomicity elements, inert gas molecules absorb an plasma energy in advance and have their large quasi-stable level energies, thereafter, giving their energies to the molecular elements, so that the molecular elements are directly excited to a higher energy states and are easily dissociated to the atomicity elements.
In the case of producing atomicity oxygen elements by dissociating the oxygen molecules with a supplied energy, the oxygen molecules has the states of O3P, O1D, O3S and so on by low energy turn. Since the oxygen molecules at each state has different activation degree, respectively, if adopted for various oxidizing reaction, it is expected that the molecules exhibits different oxidizing velocity and mechanism. If the inert gas molecules having the various quasi-stable state energies collide with the oxygen molecules to generate a plasma, the kind of the atomicity oxygen elements to be generated in the plasma may be controlled.
For dissociating the molecular elements to the atomicity elements, the inert gas molecules, not the molecular elements, absorb the plasma energy, and thereby, the useless excitations of the molecular elements are suppressed. Thus, the inert gas is introduced by the amount equal to or more than that of the molecular elements and thereby, the atomicity elements are effectively produced from the molecular elements.
In a method for forming an insulating film of the present invention, molecular silicon compound elements constituting the insulating film are introduced on a substrate surface in atomicity. The atomicity is carried out by the emission energies of the inert gas molecules absorbing the plasma energy higher than the energy required in the atomicity. Accordingly, the molecular elements are directly excited to the atomicity-state beyond the molecular excitation-state and the molecular ionization-state. The silicon substrate is oxidized as the molecular elements are oxygen molecules and is nitrided as the molecular elements are nitrogen molecules. The reactions have low activation energies, so that they are easily performed on the silicon substrate surface, not depending on the difference in their reaction temperatures on the substrate. Moreover, since the reactions cut the silicon-silicon bonds and generate the silicon-oxygen bonds or the silicon-nitrogen bonds repeatedly, the thus obtained insulating film/silicon substrate-boundary face has little dangling bonds and low boundary face-level density and thus, the insulating film having excellent insulating characteristics can be formed on the silicon substrate. Consequently, the insulating film having excellent uniform characteristics can be formed on the silicon substrate at a low temperature.